1. Field of the Invention
The present invention particularly relates to components of a modular prosthesis for implantation in a ball and socket joint cavity of a body, with particular emphasis on load-bearing ball and socket joint replacement prosthesis.
2. Prior Art
A partial shoulder prosthesis is known to the art. These have included numerous design configurations of the various components, including the head, neck, collar, locking mechanisms and stein or shaft. Prostheses for use as a total shoulder replacement are also known and essentially comprise a humeral component which is implanted in the proximal humerus and a cup or articulating member implanted in the glenoid.
A number of different types of artificial shoulder joints have been heretofore proposed, and patent and other literature describing such joints are set forth below. However, it appears that designers of some of these artificial shoulders have used various porsthesis components, from other prosthesis art.
From a general standpoint, the shoulder joint is relatively unconstrained as compared to the hip. It includes a matching ball and socket, but the ball and socket members are held in their relative positions by a "rotator cuff" which includes a heavy layer of muscles and ligaments which surround the joint, however, in addition to providing security for the ball and socket members, this muscle and ligament structure also controls the overall movement of the arm, relative to the body. Although the shoulder has been referred to casually as a "non-weight bearing joint", the compressed force acting on the shoulder joint often reaches almost full body weight, especially when the arm, when supporting the body, is raised horizontally, medically referred to as 90 degrees abduction. Also, in the course of undertaking heavy work or athletic activities, the shoulder frequently carries loads substantially greater than the body weight. Therefore the shoulder must be considered a major load-bearing joint.
Corcerning terminology, the upper arm bone is the humerus, and the ball or rounded joint member at the upper end of t ie humerus fits into a socket in the shoulder bone, or scapula. The term "glenoid" refers to the aspect of the scapula which receives the humeral ball. Accordingly, the shoulder joint is sometimes referred to as the glenohunaral articulation.
Interlocking components which have been combined into a unit hive also been utilized in shoulder prosthesis with respect to both the glenoid component and the humeral component. Such a system is known as a modular prostheses. The prior art teaches the use of a partial shoulder prosthesis, wherein the humeral component has a modular design which enabled different available sized heads to be removably connected to a humeral stem member which has been implanted in the proximal humerus. The removable connection utilizes a coupling mechanism between the head and stem members, which can comprise a Morse taper. This permits the humeral head member to be inserted into the stem member through a neck portion, the neck portion which fits into the stem member such as disclosed by Dines et al in U.S. Pat. No. 4,865,605. Alteratively, the stem may be constructed to be inserted into an opening in the head, or an intermediate element may be tapered on both sides to be inserted into both the head and the stem simultaneously.
An article "The Geometry Of The Humeral Head And The Design Of Prosthesis" by Roberts et al. in The Journal Of Bone And Joint Surgery dated July 1991 provides a dimensional analysis of approximately 30 cadaveric humeri. The summary states, "The articular surface of the humeral head is usually described as facing posteromedial, making an angle of between 16 degrees and 35 degrees wish the transepicondylar plane. At hemiarthroplasty, the articular surface also appears to be offset posteriorly with respect to the humeral shaft. Coracoid impingement may occur if this offset is not accommodated. An analysis was made of 20 cadaveric humeri using an industrial co-ordinate measuring machine. The position of the center of the head was defined with respect to the humeral shaft and transepicondylar plane. The humeral articular surface was found to be retroverted by 21.4 degrees and its center offset posteriorly by 4.7 mm.
Previous interpretation of retroversion did not take into account the posterior displacement, and this mat be of importance in improving future prosthetic design."
Hip prosthesis are also known in the art and these have also included various design configurations. Gustilo et al. in U.S. Pat. Reissue No. 32,488 discloses a neck member connecting a spherical head to the shaft member through a collar. The neck member is anteverted with respect to the collar at an angle of about 10 to 20 degrees. Stossell, in U.S. Pat. No. 4,944,764, discloses a head having a center which is offset by no more than 4.25 cm from the longitudinal axis of the shaft or stem member, to minimize damage to the patient during insertion of the prosthesis.
The U.S. Pat. No. 5,314,479, issued to Rockwood, et al teaches a four element modular shoulder prosthesis which has a three element stem which is screwed together for receiving a head for inserting into the socket cavity. The head, the fourth element, is radially symmetrical with a seating shaft that may be offset from the center of the head. The seating shaft, which has a limiting shoulder, includes a six position locking element at the end thereof, to prevent rotational movement of the head, when seated. The. locking element, however, is positioned at a major stress point and is subject to stripping or rupture.